Line cards for communication systems provide an interface for connecting communication elements to a network and typically include a number of communication ports for transmitting and receiving communication traffic to and from the network. In one configuration, an additional line card is provided for each active line card of a network element to protect the active card in the event of a fault or failure. The additional line card is typically identical to the active card and is capable of assuming and performing the functions of the active card, if the active card malfunctions, so that disruptions in communication are reduced while the active card is replaced or repaired.
An example of a redundant line card configuration is shown in FIG. 1. The arrangement comprises a first line card 3 which is normally active and a second line card 5 which is normally inactive and provides protection for the active card. Each card has a control processor (CPU) 7, 9 for controlling operations of the line card, a receiver 11, 13 for receiving control signals from a management interface 15 via a communication bus 17 and for passing control signals to the control processor, and a messaging channel 19 connected between the cards for sending line status reports and release activity messages from one card to the other.
When a data communication aspect of the active card malfunctions, the fault is detected by the management interface, and the management interface automatically controls the transfer of operations from the active to the inactive card. In particular, the management interface transmits a message to the active card via the communication bus to release activity. Once the active card releases its activity latch, the redundant card can be activated and takeover the functions of the malfunctioning card. The need for the active card to release its activity before the protection card can become active prevents both cards from being active at the same time.
One problem that has been encountered with the present configuration is that under certain fault conditions, the active card becomes entirely unreachable from the management interface, and the active card does not respond to the activity release message. In this event, the activity latch of the active card cannot be released and the redundant card cannot be activated to resume the communication functions of the active card. In this case, customers may experience significant disruptions to the communication service while the line card is down and maintenance staff are sent out into the field to rectify the fault.